In semiconductor industry, treatment of semiconductor wafers is always accompanied by operations of transfer and positioning of such wafers between the storage devices, such as wafer cassettes, and working stations of processing machines.
In the case of a stand-alone machine, manipulation with the wafers normally consists in transferring a wafer from one cassette to the stand-alone machine with the subsequent transfer of the treated wafer from the machine to another cassette. In some cases, after treatment in the stand-alone machine, the treated semiconductor wafers are returned to the slots of the same cassette. Such an operation is associated with a more complicated mapping procedure than in the case of two cassettes. This is because in the second case the sensor system of the robot has to detect and remember all Z-positions and thicknesses of the wafers in the slots of the cassette which are filled with the wafers and which are free for insertion of the treated wafers. The same situation occurs in the case of treating the wafers in cluster machines with the difference that the robot arm manipulates the wafers between working stations of the cluster machine and a single or several cassettes.
Semiconductor wafers are generally racked or mounted vertically on their edges and stacked horizontally in plastic cassette carriers. Each carrier contains many parts next to each other with a small separation between each part. Detecting the edge of a wafer or disk permits accurate positioning information to be obtained allowing automated handling equipment to access and remove individual parts for processing without damaging adjacent parts in the carrier.
The wafer processing machines are equipped with special sensors, known as mapping sensors, which detect improperly aligned parts, missing parts, double-wafers or double-disks (i.e., wafers or disks mounted with no spaces between them) alerting the technician or automated equipment to possible defective parts, or to pass over the defective parts to prevent further processing. However, sensing of the extremely thin, compoundly curved edges of semiconductor wafers has, until now, represented a significant challenge in developing edge sensing devices capable of rapidly and accurately sensing these edges.
Sensing devices currently being used to detect semiconductor wafers include a “through beam”, which is a beam that is emitted from a light source to a light-receiving element of the sensor and is interrupted or blocked by a peripheral edge of the wafer when it is transferred from one operation position to another. However, the through-beam type sensors are difficult to align and, generally, must be dedicated to a cassette for specific parts. Further, the through beam is unable to detect “double-stacking”; i.e., where two wafers or disks have inadvertently been mounted in the parts carrier so that their adjacent faces are in contact. Double stacking invariably causes defects, such as scratches, on the precision surfaces of these products resulting in lower process yields and increased costs due to rejected parts. Ideally, early detection of double stacking is desirable to prevent further costly processing of these defective parts and to help identify which process step is the cause of the double stacking.
Through-beam detection of parts will not be able to identify double-stacked or cross-slotted parts in those carriers where the parts are tilted or slightly askew in their slots. As semiconductor wafers are very thin and the slot in the carrier is generally of a design that does not support the wafer equally around its circumference, the wafer will sit slightly tilted in that slot. The tilted wafer or disk presents a wider profile to the through-beam than that of a perfectly aligned wafer. This wider profile may be mistakenly interpreted by the through-beam system as a double stacking occurrence.
Another process-related error, which may result in product defects, is “cross-slotting.” Cross slotting occurs when a semiconductor wafer or magnetic disk is positioned in the parts carrier such that one edge of the wafer or disk in contact with the carrier is in the wrong retaining slot in the carrier.
Some systems for detecting positions of wafers in the cassettes are based on the use of so-called proximity sensors, which involve the use of a fiber optic light guide, brought in close proximity to the position where the perimeter edge of the part is anticipated to be. These stackers require precise alignment for docking into a receiving bay. The fiber optic sensor directs light towards the anticipated location of the perimeter edge of the top disk in the stacker. The fiber optic sensor detects the presence of the top wafer in the stacker by receiving the reflected light back into the fiber optic cable with the reflected light being sensed by an optical sensor. However, this system requires that the terminal end of the fiber optic cable be in extreme close proximity to the edge of the top wafer and that the incident light from the optic cable impinge at a 90° angle to the tangential surface of the edge of the wafer. In addition, the edge of the wafer must be thick enough so as to present as flat a surface as possible to the fiber optic light in order to provide enough surface to reflect back a sufficient amount of light to trigger the sensor. Thinner wafers having a compoundly curved edge will not reflect sufficient light directly back to the fiber cable and, therefore, the sensor will not detect the disk. In order to maintain such close proximity, the sensor is rigidly affixed to either the stacker or the receiving bay, thus precluding its use for rapid parts counting. This unreliability could result in process throughput deterioration because the fiber optic sensor erroneously senses there are no more parts to process causing the process to stop. Alternately, the stacker may continue indexing upward despite the top wafer not having been sensed and removed causing a “double-wafer” to occur as the unsensed wafers falls back onto the next wafer being indexed. In either case, such unreliability will require that an operator or technician be present to continually monitor production processes, thus negating the reasons for installing automated parts handling. This scheme is further limiting since the close proximity and the 90° angle of incidence required by the device precludes rapid scanning across the length of the carrier for a rapid parts count.
The problems inherent in through-beam sensors and in proximity sensors were partially solved by the device and method described in U.S. Pat. No. 5,504,345 issued in 1996 to H. Bartunek, et al. The Bartunek, et al. device provides a wafer edge detection system having a converging dual-beam optical sensor for detecting the presence of small, specular surfaces, particularly small radius curved surfaces. The sensor comprises at least two light sources, preferably lasers, and at least two light detectors. Alternately, a single light source with its light beam passed through the appropriate optics may have its beam split to create at least two light beams. Further, a single light detector may be used together with the appropriate optics such that the reflected light is directed by the optics to the light detector. The light sources, or alternately light beams emanating from a light source, are spatially oriented such that the focal point of the converging light beams defines a focal or inspection plane and converges at a single point external to the device. When the specular or reflective surface to be detected interrupts the beam at or near the focal point of the light sources, it causes the light to be reflected backwards towards the sensor for direct detection by the light detectors or for indirect detection where the appropriate optics direct the reflected light to at least one light detector. The light detectors, or the optical path for indirect detection, are spatially arranged to permit detection of the reflected light even though the surface to be detected is curved or presents a reflecting angle, or angle of incidence, deviating significantly from 90°.
However, the device of Bartunek et al. does not solve some other problems associated with the use of known mapping sensors. One unsolved problem consists in generation of false signals, e.g., when the sensor generates a signal that the cassette slot is occupied, while it is free. This problem is associated with the loss of a valuable and expensive time of a working cycle. Another more serious problem occurs in generation of a signal stating that the cassette slot is free while it is occupied by another wafer. Such false signal may cause serious damage to the equipment by inserting an expensive treated wafer into the occupied slot of the cassette. This operation may result in a crush or even in more serious and expensive damage. Another general problem in connection with the use of known mapping sensors consists in that, in order to provide reliable operation of the sensors, it is necessary to in crease the power of laser light sources to the level unacceptable for operation in open spaces where the exposed laser light becomes dangerous for the operator. In other words, the intensity of the laser light becomes higher than the sanitary norms specified by respective FDA standards. Increase in the power of laser light sources is associated not only with hazard to the operator's health but also with intensification of light reflected from the inner walls of the cassette, which results in generation of many false signal. Some of these intensified false signals may reach or even exceed the level of sensitivity of the mapping sensor, which in this case generates a false signal. A third problem consists in that practically all conventional mapping sensors used in the semiconductor production field operate with difractionally-limited light beams having transverse dimensions comparable with the width of a notch on a disk. It is known that almost all wafers used at the present time are provided with small V-shaped cutout portions (hereinafter “notches”). When the beam of the mapping sensor with a narrow cross-section coincides with the position of the notch, it may generate the aforementioned false signal of the type indicating that the cassette slot is free. This is because the beam reflected from the surface of the notch may have a direction different from the one reflected from the peripheral edge of the wafer, or may have intensity of light signal below the threshold of the sensor.
Another mapping sensor system is described in U.S. patent application Ser. No. 09/944,605 filed by the same applicants on Sep. 4, 2001. This known mapping system is shown in FIG. 1A, which is a schematic three-dimensional view of a mapping sensor system in conjunction with an end effector of a mechanical robot. As shown in FIG. 1A, the end effector 20′ has a mounting plate 22′ attached to a robot arm (not shown). The plate 22′ supports a stepper motor 24′. The output shaft 28′ of the stepper motor 24′ is connected through a spring (not shown) to an elongated finger 29′ that slides in a central longitudinal slot 30′ of the plate 22′ and supports a first wafer gripping post 32′, pivotally supports two L-shaped fingers 34′ and 36′ with a second and third wafer gripping posts 38′ and 40′ on their respective ends. The mounting plate 22′ in combination with the first sliding finger 29′ and two pivotal fingers 34′ and 36′ forms the end effector of the robot arm which is thin enough for insertion into a wafer-holding slot 42′ of a wafer cassette 44′. It is understood that the aforementioned end effector was shown only as an example, and that this can be a wafer-handling system for operation with the wafer cassette that stores circular wafers W′ in narrow slots.
In the system of FIG. 1A, the mapping system consists of a light source 46′ such as a laser diode and a light-receiving element such a photodiode 48′. The laser diode 46′ may be of ML1016R-01 type produced by Mitsubishi Electric Corp., Japan. The light beam B′1 generated by the laser diode 46′ is focused on the wafer edge with the use of a special objective or a spherical lens (not shown) which produces a beam of a round cross section. The photodiode 48′ may of a conventional type, which is sensitive to the light of laser diode reflected from the peripheral edge E′ of the wafer W′. The wafer W′ has a notch N′ or flat on its peripheral edge E′. It can be seen that the mapping system is mounted on the plate 22′ in front of the cassette 44′ which is convenient for mapping of the wafer positions in the cassette 44′.
In operation, the light source 46′ emits a light beam B′1 which is focused on the edge E′ of the wafer W′, e.g., in the slot 42′ of the cassette 44′. If the wafer W′ is present in the slot 42′, the beam B′2 reflected from the edge E′ of the wafer W′ is sensed by the photodiode 48′. The latter produces on its output a signal sent to the control unit (not shown) of the end effector 20′. However, the mapping system of FIG. 1A with a single beam B′1 focused on the edge E′ will not produce a signal if the beam B′1 falls onto the notch N′.